Nematic liquid crystals are centrosymmetric media. Thus, electrooptic effects such as the dielectric Freedericksz transition are quadratic in nature (i.e., subject to the square of electric field) and as a result are nonsensitive to electric field polarity. However, nematic liquid crystals are always bounded and have surfaces at which the centrosymmetric order can be broken in which case surface polarization of the liquid crystal arises.
A homeotropic liquid crystal cell including alignment layers having identical compositions is discussed in O.D. Lavrentovich et al., "Electrooptical Effect Induced by Surface Polarization in a Nematic Liquid Crystal", Zh. Tekh. Fiz. 60, 208-211 (1990)/Sov. Phys. Tech. 35, 127-129 (1990) and O.D. Lavrentovich et al. "Surface Polarization and Domain Structures in Thin Nematic Layers", Mol. Cryst. Liq. Crystl, 192, 239-243 (1990). These articles disclose cells having two identical alignment layer coatings of silicon elastomer and cells having two identical alignment layer coatings of silicon elastomer and lecithin. In other words, both substrates are coated with the same alignment layer material. In these cells, placed between two crossed polarizers an electrooptical effect was observed, i.e., in the absence of electric field no light was transmitted, and upon application of an electric field the cell transmitted light. However, such cells do not produce a unipolar electrooptical effect, i.e., an optical instability for only one polarity of electric field.
The liquid crystalline electrooptical light modulating device of the present invention overcomes the inability of previously proposed symmetric liquid crystal systems to produce a unipolar electrooptical effect. By bounding a nematic liquid crystal by two substrates which are each treated with an identical alignment layer, as in the previously proposed liquid crystal cells, the directions of vectors representing a surface polarization "P.sub.s " of the liquid crystal adjacent each substrate are antiparallel to each other. In other words, the P.sub.s vectors at one substrate point in opposite directions than the P.sub.s vectors at the other substrate. Coupling of P.sub.s with the electric field E leads to a linear contribution (-P.sub.s E) to the free energy of the system. This coupling results in a destabilizing torque of the liquid crystal at the substrate surface when the direction of the P.sub.s vectors and E are antiparallel to each other and in a stabilizing torque of the liquid crystal when P.sub.s and E are parallel to each other. In a symmetric cell with P.sub.s .noteq.0 there is always a destabilizing moment at one of the two plates and deformations in the optical axes of the liquid crystal arise regardless of the direction of E. Reversing the polarity of E only changes the plate at which the deformations occur and is not known to result in a unipolar electrooptical effect. Thus, a unipolar electrooptical effect is not observed when using identical alignment layer treatments.